Slippery slope

 A slippery slope is a metaphor sometimes used to oppose a change that may seem good in itself, but might lead to further changes that we would regret.  The metaphor is that we are perched on a high plateau, and any attempt to move higher might (even if initially successful), cause us to plunge into the abyss.

The perspective is different if instead we're at a low place, trying to get higher, but we keep backsliding because the slope is slippery.